If you ride a fixie then this website will help you choose the right gear ratio and tell you how many bald spots are going to appear on your back tire. Also, if you have an iphone, I have put a link to where you can get the app for that too.
Helpful website: http://software.bareknucklebrigade.com/rabbit.applet.html
A helpful geek built this for your iphone: www.jpmartineau.com/iphone/bicycle-gear-calculator. to install it: www.tinyurl.com/bikegears <= will open iTunes. Get it while it's free. Hope this will help to solve my-gear-ratio-is-bigger-than-yours disputes.